Fecal stanols provide a proxy of population change by identifying variations in the amount of trace human waste products retained in sediment. We used fecal stanol data from Horseshoe Lake, Illinois, as a population proxy to evaluate the role of flooding, droughts, and environmental degradation in Cahokias demographic decline. We find that both Mississippi River flooding and warm season droughts detrimental to maize agriculture occurred at ca. 1150 CE, shortly after Cahokias population maximum as identified by fecal stanol concentrations. Fecal stanols then decrease steadily to a minimum at ca. 1400 CE, suggesting environmental stressors of the mid-12th century played a significant role in Cahokia's decline. Because everybody poops, we can use human waste to assess the impact of environmental events on a population.